Society continues to move into an age of electronic information as the creation and exchange of electronic data steadily increases. Electronic data is increasingly used for such endeavors as work, education, entertainment and communication. As the use of electronic data increases and expands, so has demand for lower cost and higher performing electronic data devices.
Additionally, the demand for mobile electronic data devices has also increased. In addition to being a society of electronic data, society is also becoming more of a mobile society. Not only is there a demand for electronic data at home or work, but the mobile society also demands access to electronic data without the limit of wireline connections. For example, users want the ability to wirelessly use a laptop throughout their home but yet have access to electronic information as if using a desktop computer. Further, users want access to electronic data through cellular telephones or other mobile computing devices such as personal digital assistants (PDA).
Unfortunately, exchanging electronic data wirelessly presents such problems as interference, security, delay and quality. Several modulation methods have been developed to improved the wireless transfer of electronic data. One such method is orthogonal frequency division multiplexing (OFDM). OFDM is a method of wirelessly transmitting and receiving electronic data at a high data rate and low complexity even over hostile channels.
A preferred configuration for OFDM transceivers includes a direct conversion transmitter and a zero or low intermediate frequency (IF), receiver. A direct conversion transmitter transmits data at a carrier frequency provided by a clock, or more specifically, a local oscillator without using an intermediate frequency. Similarly, a zero intermediate frequency receiver is also known as a direct conversion receiver since it uses a local oscillator that coincides with an incoming carrier frequency giving an intermediate frequency of zero. A low intermediate frequency receiver, on the other hand, uses a mixer to convert the incoming carrier frequency signal to a lower fixed intermediate frequency for further processing. Typically, the mixer mixes a signal from a local oscillator with the incoming carrier frequency resulting in the intermediate frequency. The local oscillator usually operates at or near the incoming carrier frequency with the intermediate frequency being a difference between the local oscillator signal and a data signal frequency.
Though preferred, this OFDM configuration creates a difficult local oscillator (LO) design since a different LO frequency is needed for the transmit and receive modes. Typically, the frequency difference for the transmit and receive modes is the intermediate frequency.
The different LO frequencies for transmitting and receiving may be achieved by using a single voltage controlled oscillator (VCO) and a single frequency synthesizer that is reprogrammed during the transmit and receive modes. The frequency synthesizer may be a frequency divider. Unfortunately, the time needed to switch from one frequency to another is too great for the strict timing requirements required between the transmit and receive modes.
In another configuration, two VCO's and two frequency synthesizers may be used to produce the two required frequencies for transmitting and receiving. In this configuration, one combination of the VCO's and synthesizers may be used for one frequency and another combination of VCO's and synthesizers may be used for the other frequency. Though there is no timing problem as in the first configuration, this configuration may create an adverse output spectrum due to cross talk between the two VCO's.
Accordingly, what is needed in the art is a local oscillator capable of producing a clean signal and a fast switching signal for an OFDM transceiver.